Avoid returning original macro if expansion fails.

Closes #11692. Instead of returning the original expression, a dummy expression
(with identical span) is returned. This prevents infinite loops of failed
expansions as well as odd double error messages in certain situations.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libsyntax/ext/trace_macros.rs b/src/libsyntax/ext/trace_macros.rs
index 4189ea6a967..db2c9dcddb6 100644
--- a/src/libsyntax/ext/trace_macros.rs
+++ b/src/libsyntax/ext/trace_macros.rs
@@ -33,7 +33,7 @@ pub fn expand_trace_macros(cx: &mut ExtCtxt,
         cx.set_trace_macros(false);
     } else {
         cx.span_err(sp, "trace_macros! only accepts `true` or `false`");
-        return base::MacResult::dummy_expr();
+        return base::MacResult::dummy_expr(sp);
     }
 
     rust_parser.bump();